1. Field of the Invention
The present invention relates to a driving device for a vibration type actuator, such as an ultrasonic vibration driven motor and, more particularly, to a driving device for a vibration wave actuator which can execute a driving operation with high precision without using a vibration detection sensor.
2. Related Background Art
A vibration type (vibration wave) actuator, such as an ultrasonic vibration driven motor, comprises a vibration member, which is prepared by adhering and fixing piezoelectric elements to one surface side of a metal elastic member, e.g., an elastic member formed in an elliptic shape. The piezoelectric elements are subjected to a polarization treatment, and when AC electric fields having a predetermined phase difference are applied to the piezoelectric elements, a traveling vibration wave is generated on the other surface side of the elastic member.
If the surface, on which the traveling vibration wave is generated, of the elastic member is defined as a driving surface, when a movable member is brought into contact with the driving surface via a compression means, the movable member is driven by a frictional force therebetween. The movable member maybe a rotor for extracting an output, a sheet such as paper, or the like.
Such a vibration wave actuator has characteristics suitable for high-precision alignment. More specifically, since the movable member is always in contact with the elastic member of the vibration member under pressure, when the driving operation of the vibration member is stopped, the movable member is immediately stopped.
On the other hand, the following control methods for driving the vibration wave actuator are available:
(1) a method of controlling the speed on the basis of a voltage in such a manner that a sensor phase is provided in a portion of the piezoelectric elements of the vibration member, and the frequency of a driving voltage is set to be equal to the resonance frequency of the vibration member, as disclosed in Japanese Laid-Open Patent Application No. 63-167680; PA1 (2) a method of directly controlling the speed on the basis of a frequency, as disclosed in Japanese Laid-Open Patent Application No. 59-156168; and PA1 (3) a method wherein a sensor phase for detecting a vibration state is provided in the piezoelectric elements of the vibration member, and the frequency of a driving voltage is controlled according to the detected value, as disclosed in Japanese Laid-Open Patent Application No. 63-1379.
Since the frequency-speed characteristics of the vibration wave actuator have characteristics wherein the speed quickly changes on the low-frequency side and slowly changes on the high-frequency side with a resonance point as a boundary, therebetween a frequency region on the high-frequency side of the resonance point is utilized in the control method. Therefore, in order to increase the speed, the frequency is decreased to approach the resonance point, and in order to decrease the speed, the frequency is increased to be separated away from the resonance point.
However, since the control method (1) requires a sensor, the cost is increased. An actuator controlled by this method also is susceptible to noise from the sensor. When low-speed control is executed, control becomes unstable due to a small driving voltage. In addition, since the control method must cover a range from low speed to high speed by changing the voltage amplitude alone, the amplitude setting resolution must be increased, resulting in high cost as a whole.
In the control method (2), since a constant the amplitude of a voltage is set if the frequencies are not adjusted in units of actuators upon starting of these actuators, then the speeds of the actuators upon starting do not coincide with each other, and the actuators perform considerably different operations when they perform movements over a small distance. When the actuator is driven near the resonance frequency, it is susceptible to disturbance. The frequency of the driving voltage is shifted from the resonance point to the low-frequency side by a slight disturbance due to nonlinear vibration characteristics, and as a result, the actuator is abruptly stopped.
Since control method (3) requires a sensor as in control method (1), this results in high cost, and the actuator controlled by this method is susceptible to noise from the sensor.